Tooling having advantageously located heat transfer channels

ABSTRACT

Processes for providing enhanced thermal properties of tooling, particularly metal and metal/ceramic molds, made by solid free form fabrication techniques, such as the three dimensional printing process, and the tooling made by these processes are disclosed. The methods of enhancing thermal properties include incorporating integral contour coolant channels into the mold, adding surface textures to the coolant channels, creating high thermal conductivity paths between the surfaces and the coolant channels, and creating low thermal inertia regions in the mold.

This application is a continuation of application 09/109,462, acontinued prosecution application filed on Aug. 5, 1999, originallyfiled on Jul. 2, 1998, now U.S. Pat. No. 6,112,808 which is a divisionalof 08/551,012, filed on Oct. 31, 1995 now U.S. Pat. No. 5,775,402,

This invention was made with government support under grants numberDDM-8913977 and 9215728-DDM awarded by the National Science Foundation.The government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to the field of tooling manufacture by layeredfabrication techniques and more particularly to three dimensionalprinting of metal and metal/ceramic molds.

BACKGROUND OF THE INVENTION

Metal molds for forming processes such as injection molding, blowmolding, die casting, forging, and sheet metal forming are currentlymade using manufacturing techniques such as machining, EDM, casting andelectroforming. (K. Stoeckhert (ed.), “Mold Making Handbook for thePlastics Engineer,” Oxford University Press, New York, N.Y. 1983.) Thecreation of the tool is a multi-step process involving a variety ofmanufacturing techniques. The mold is created by removing material froma block of metal, usually a tool material such as tool steel. Typically,a block of annealed tool steel is first rough machined to near-netshape. The near-net shape tool may then be hardened with an appropriateheat treatment cycle to obtain the desired final material properties.Final dimensions are obtained by finish machining, grinding or EDM ofthe mold pieces. Final finishing may also occur before the hardeningstep. Selected tool surfaces are then modified as required. Matingsurfaces are typically ground to provide adequate sealing. Surfaceswhich require additional hardness or abrasion resistance can be treatedby techniques such as nitriding, boriding, plating or ion implantation.(K. Stoeckhert (ed.), “Mold Making Handbook for the Plastics Engineer,”Oxford University Press, New York, N.Y. 1983.)

Alternate techniques exist for creating the near-net shape tool pieces,such as casting or electroplating. Near-net shape tool ingots made bycasting are produced using established casting techniques. Aftercasting, the metal perform must be finished using the additional finishmachining or EDM processes described above. Tool performs made byelectroforming are produced by electroplating a metal, typically nickel,onto an appropriately shaped mandrel. After plating to sufficientthickness, the tool is removed from the mandrel. Although the face ofthe tool is completely defined by the electroforming process, otherportions of the tool must be created using other processes. A backingmaterial, such as metal-filled epoxy, must be added to the rear of thetool and machined to the appropriate shape before the tool can be used.An alternate method of producing metal tools, the Tartan Tooling method,is described in U.S. Pat. No. 4,431,449 and U.S. Pat. No. 4,455,354. Inthis method, metal powder is packed around a negative of the shape to beproduced and bonded with a polymeric material. The negative can beproduced by any convenient means. The bonded powder green part is thenfired to remove the polymer and to partially sinter the part. Finally,the porous sintered part is infiltrated with a lower melting point alloyto fill the residual porosity, producing a fully dense metal tool. Toolsproduced by the Tartan Tooling method have fewer finishing requirementsthan near-net shape performs made by other processes, but some finishingis usually required.

Metal molds, tools and dies produced by the above techniques must meet avariety of performance requirements. These requirements are determinedby the type of forming processes the mold will be used for. Tools usedfor injection molding, for example, must remove heat from the injectedpart to cool it and freeze its shape. The transfer of heat away from themolten plastic directly affects part cycle time, dimensional accuracyand material properties. (K.Stoeckhert (ed.), “Mold Making Handbook forthe Plastics Engineer,” Oxford University Press, New York, N.Y. 1983.)Molds with poor heat transfer characteristics require longer waitingperiods before the plastic part has solidified enough to be ejectedwithout damage, thus increasing cycle time. Molds in which the polymerfreezes non-uniformly due to uneven heat removal can result inanisotropic shrinkages across the part, causing part warpage and loss ofdimensional control. Additionally, residual stresses are incorporatedinto the plastic part during uneven cooling, having a detrimental effecton the material properties of the part. Injection molds typicallyincorporate fluid coolant channels to increase the rate at which heatcan be removed from the injected plastic. (R. G. W. Pye, “InjectionMould Design,” 4th ed., Longman Scientific and Technical, Essex,England, 1989.) The coolant channels are incorporated into the moldusing traditional machining or EDM techniques. The layout of the coolantchannels is dependent on part geometry and the specific limitations ofthe processes used to create the channels.

Cooling of a tool can be effected in one of two ways. For smaller partsin which a tooling insert mold 1 is used, as shown in FIG. 1, thecoolant channels 2 are located in the backing plate assembly 3, whichalso provides the majority of the mechanical support necessary to resistmold deflection during the injection cycle. (H. Gastrow, “InjectionMolds: 102 Proven Designs,” Hanser Publishers, Munich, 1983.) Channelsare not directly incorporated into the insert itself because of theadditional expense and fabrication time. Also, the size of channels forinsert molds may be prohibitively small, making fabrication difficult.For larger, non-insert type molds 4, shown in FIG. 2, the coolantchannels 5 are directly incorporated into the mold. In both cases, thelocation and configuration of the coolant channel layout is a compromisebetween ideal cooling and practical limitations. For insert molds, thebacking plate coolant channel layout is not tailored to a specificinsert mold but is designed with a generic layout, and therefore cannotmeet the ideal coolant needs of a particular insert geometry.Additionally, the heat flux transferred from the mold insert to thecoolant plate must pass through the gap between the insert and platesurfaces. Although this gap is usually filled with a heat conductivegrease or other material, the overall heat transfer is lessened. Forlarger molds 9, shown in FIG. 3, the coolant channels are usually arraysof straight cylindrical holes 10 which are made using standard machiningprocedures. The channels are incorporated into the mold as an additionalmold fabrication step after the mold cavity or core has been defined.The actual layout of the channels is limited by the shape of the cavityor core, in addition to fabrication constraints. The simple straightcylindrical channels cannot follow the complex contours of a typicalcavity or core, resulting in uneven cooling of the mold surfaces. Also,the number and placement of the channels cannot be allowed to compromisethe mechanical integrity of the mold. Also, rework of coolant channels,as might be required if the initial configuration does not performadequately, becomes increasingly difficult as more mold material isremoved. Improper layout of the channels may require the entire mold tobe scraped.

The cylindrical holes are usually created by drilling. The cylindricalshape of the coolant channel is therefore a consequence of themanufacturing technique used to create it and not because it is theideal shape for heat transfer purposes. The internal surface area of thechannel, which directly effects the overall heat transfer from moldmetal to coolant, is limited by this requisite cylindrical shape.Increasing channel diameter or the number of channels in the mold areways to increase the effective channel wall surface area, but thesetechniques are limited by mold geometry. Other methods of increasingheat transfer which are commonly found in heat exchanger design, such asfinned or textured surfaces, are not readily adaptable to mold coolantchannels made by conventional means.

Most tool steel alloys are tailored for high strength, hardness andtoughness in order to survive millions of injection cycles. Tool steelstypically have fair to poor thermal conductivity values compared toother softer tooling alloys, specifically copper alloys, although thecopper alloys cannot match the strength and hardness of tool steel.Since the components of injection molding tools are made almostexclusively from a single material, a compromise has to be made withregard to either high strength or high thermal conductivity properties.In some very complicated, multi-component tools, different parts of thetool can be made from different alloys. For example, a tool steel moldcore 15 with a high aspect ratio can be drilled out and a dowel pin 16made from high thermal conductivity metal can be inserted to provide anenhanced heat transfer path to an adjacent coolant channel 17, as shownin FIG. 4. Tool modifications of this type can only be used in a limitedset of geometries and add to the complexity of fabrication and cost ofthe tool.

Most injection molds are actively cooled so that heat will be removedquickly from the injected plastic, allowing for faster plasticsolidification and decreased cycle times. The coolant fluid supplied tothe mold is at a preset temperature. Regardless of the temperaturechosen, however, a compromise invariably results between finished partquality and cycle time. If a high coolant temperature is chosen, thefinished part will have low internal stress and good dimensionalaccuracy. Cycle time, however, is lengthened due to the slow cooling. Ifa low coolant temperature is chosen, the cycle time is reduced due tofaster cooling, but the rapid and potentially non-uniform freezingresults in parts of lesser quality. A method for simultaneouslyachieving low cycle time and high part quality would involve rapidlyheating the mold by electrical resistance just before injection, andthen rapidly cooling the mold after injection to decrease cycle time.(B. H. Kim, “Low Thermal Inertia Injection Molding,” MIT Ph.D. Thesis,1983.)

Metal molds for forming processes can also be created directly from acomputer model using processes that construct objects in layers, such asthe three dimensional printing process described for example in U.S.Pat. Nos. 5,204,055, 5,340,656, and 5,387,380. In a typical applicationof this process, the mold is created by spreading a powder layer using aroller within a confined region as defined by a piston and cylinderarrangement. A water soluble polymer is then deposited at specificregions of the layer as determined by a computer model of the mold. Thewater soluble polymer acts to bind the powder within the layer andbetween layers. This process is repeated layer after layer until alllayers needed to define the mold have been printed. The result is a bedof powder which contains within it a polymer/metal composite of thedesired geometry. Unbound powder temporarily supports unconnectedportions of the component, allowing overhangs, undercuts and internalvolumes to be created. Next, the entire bed is heated to a temperaturesufficient to cure the polymer and bind the printed regions together.The mold is then removed from the loose, unprinted powder to provide agreen metal mold. A variety of post-processing options exist fortransforming the three dimensional printed green metal mold into a fullydense, all metal part. The polymeric binder must first be removed bythermal decomposition in a process called debinding. One method ofdebinding is accomplished by firing the part in a non-oxidizingatmosphere at temperatures in excess of 500° C. (R. M. German, “PowderInjection Molding,” Metal Powder Industries Federation, Princeton, N.J.,1990.) After debinding, additional processing is performed to fullydensity the part. For example, the debound part is directly sintered tofull density using a heating schedule appropriate to obtain fulldensification by sintering. In this approach, debinding and sinteringcan occur in a single continuous operation. Another means of obtainingfull density is to sinter the metal skeleton after debinding such thatonly part of the porosity is eliminated. Again, debinding and sinteringcan occur in a single continuous operation. In a second firingoperation, the metal skeleton is infiltrated with a Lower melting pointalloy, thereby filling the residual porosity with infiltrant metal. Theamount of sintering required before infiltration will depend on theinfiltration alloy and infiltration temperature. The sintered metalskeleton must be strong enough to resist the capillary forces induced bythe liquid infiltrant. Typically, sintered densities in excess of 65% oftheoretical are sufficient. Typically, the green part would be printedat about 60% theoretical density and sintered to 65% theoreticaldensity.

SUMMARY OF THE INVENTION

The current invention provides tooling, particularly metal andceramic/metal molds, produced by solid free form fabrication techniqueswith unique thermal features and properties. The invention isparticularly suited for use with three dimensional printing techniques.

According to one aspect of the invention, molds are made with internalcontour cooling channels which are incorporated into the mold during theprinting process. There are no restrictions on the number, size, shapeand routing of these channels. The internal channels can be printed soas to be completely open, or they can be filled with a porous material.The shape and texture of the cooling channel walls can be optimized forhigh heat transfer by increasing channel surface area and modifyingchannel surface texture. For example, the channels can incorporatethermal structures such as fins to increase surface area to enhance heattransfer. The surface texture of the internal channels can be controlledby the selection of the powder and binder used to print the green part.

During fabrication of the mold, the cooling channels can be left openduring the infiltration step by creating a pressure differential betweenambient and the infiltrant melt front, such as by placing the mold onstilts of a predetermined minimum height through which the infiltrantrises by capillary action. Also, by printing an “infiltration stop”material in appropriate regions of the tool volume, the cooling channelscan be left open after the infiltration step of post-processing.

In another aspect of the invention, internal cellular structures,preferably in combination with cooling channels, are created beneath themold surface during printing. The thermal inertia of the mold is reducedby such a cellular structure, allowing for rapid thermal cycling of themold.

The invention also provides enhanced thermal properties created by theselective deposition of high thermal conductivity materials, therebyincreasing heat transfer from the tool surface to the coolant channels.For example, high thermal conductivity paths can be created between themold surfaces and the coolant channels. Selective printing of highthermal conductivity materials additionally allows the creation ofgradual compositional gradients within the tool, thereby minimizinglocal stresses caused by the local variation in coefficient of thermalexpansion and increasing the overall toughness of the tool.

Buried pockets of a high thermal conductivity material can also beprovided by incorporating a pocket open on one end during the printingprocess, removing unbound powder from the pocket through the open end,sealing the open end, and infiltrating the part with an infiltrant of ahigh thermal conductivity material to fill the pocket.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic illustration of a prior art insert mold andbacking plate incorporating coolant channels;

FIG. 2 is a schematic illustration of a prior art mold incorporatingcoolant channels;

FIG. 3 is a schematic illustration of a prior art mold showingcylindrical coolant channels;

FIG. 4 is a schematic illustration of a prior art mold including a highthermal conductivity dowel pin;

FIG. 5 is a schematic illustration of a mold incorporating contourcoolant channels according to the present invention;

FIG. 6 is a schematic illustration of a mold incorporating heat transferelements inside the coolant channels according to the present invention;

FIG. 7 is a schematic illustration of a mold incorporating further heattransfer elements inside the coolant channels according to the presentinvention;

FIG. 8 is a schematic illustration of a mold incorporating a highthermal conductivity material according to the present invention;

FIG. 9 is a schematic illustration of a mold having a reduced thermalmass from a cellular truss structure according to the present invention;

FIGS. 10A, 10B, and 10C are schematic illustrations of a moldinfiltrated on stilts;

FIG. 11 is a schematic illustration of a multiple reservoir system foroperation of a mold for rapid thermal cycling;

FIG. 12 is a schematic illustration of a further multiple reservoirsystem for operation of a mold for rapid thermal cycling;

FIG. 13 is a schematic illustration of tooling containing buried pocketsof a high thermal conductivity material;

FIGS. 14A, 14B, 14C, and 14D are schematic illustrations of a processfor forming the buried pockets of FIG. 13;

FIG. 15 is a schematic illustration of a system for removing powder fromthe channels of a printed part; and

FIGS. 16A, 16B, 16C, and 16D are schematic illustrations of a furtherinfiltration embodiment;

FIG. 17 is a schematic illustration of an infiltration stop;

FIG. 18 is a schematic illustration of a thermal inertia backingmaterial;

FIG. 19 is a schematic illustration of a further embodiment of amultiple reservoir system for operation of a mold for rapid thermalcycling; and

FIG. 20 is a schematic illustration of a three-dimensional trussstructure forming a reduced thermal mass portion of a mold according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for the enhancement of thermal propertiesof tooling used in a variety of processes, such as injection molding,thermoforming, blow molding, die casting, resin transfer molding,reaction injection molding, sheet metal forming, transfer molding,forging, and others. The tooling is manufactured by a solid free formfabrication technique (SFF) from a computer aided design (CAD), as inthe three dimensional printing process.

In a first embodiment, as shown in FIG. 5, a mold 8 is fabricated withintegral conformal cooling channels or passages 6 incorporated directlyinto a mold 8. The channels advantageously may follow the contour of, orconform to, the surface of the mold. These conformal channels can be ofarbitrary shape, size, and complexity. They can be made in any crosssection and are not limited to round sections as are drilled passages.In some cases, non-circular cross sections arranged with a greaterperimeter length close to the mold surface are advantageous to presentmore coolant close to the surface of the molding cavity. For example,the cross section can be elliptical with the long axis generallyparallel to the mold surface. The channels are open on both ends andextend from one edge of the mold to another edge to provide an inlet andan outlet for a coolant to flow therethrough during molding of a part.The channels can take a variety of paths through the mold and caninclude branched or interconnected configurations. Many channels can bespaced close together to provide more cooling. Such techniques are wellsuited to both the cavity side of an injection mold and to the coreside. In fact, in the absence of such conformal cooling, cores areparticularly difficult to cool.

The design of the channels is written into a CAD model of the moldduring the mold design process. During fabrication of the mold, as bythree dimensional printing, a powder material is deposited in layers,and a polymer or inorganic binder is selectively printed on each layer.The powder material may include stainless steels, such as the 400 seriesand particularly 420, tool steels such as H13 or S7, or carbides, suchas tungsten carbide, titanium carbide, or tantalum carbide. The coolingchannels are automatically formed as unbound regions during theprinting. After process completion and curing of the polymer binder, theunbound powder is removed from the mold, leaving behind the desirednetwork of channels. The mold is then debound and either sintereddirectly to full density, or presintered and then infiltrated. In thismanner, channel shape and layout can be optimized for heat transferwhile avoiding adverse effects to the mechanical integrity of the mold.Both mold inserts and large molds can have cooling channels incorporatedtherein without compromise to fabrication time.

In order to be effective as conformal cooling channels, the coolingchannels must be placed close to the surface of the tool. If thechannels are placed far away, they will still withdraw heat. However,there will be a significant temperature difference between the surfaceof the molding cavity and the coolant. Further, this temperaturedifference depends on the cycle time, the temperature of the injectedplastic, and other process parameters. Thus, as these parameters change,the temperature of the molding cavity changes, an undesirablecircumstance.

An approximate value for the maximum distance that a conformal coolingchannel may lie from the surface of a molding cavity may be derived byreference to the literature on transient heat transfer. (See, forexample, W. Rhosenow and H. Choi, Heat Mass and Momentum Transfer,Prentice-Hall, 1961, p. 121.) For example, if a conductive body is at auniform temperature and its surface is suddenly raised in temperature byan amount ΔT, then heat is conducted into the body. The temperaturerises such that at time t, the temperature at a distance (αt)^(½) fromthe surface has risen by approximately ΔT/2, where α is the thermaldiffusivity of the body. Thus, if the cooling channels are placed closerto the surface of the molding cavity than (ατ)^(½) where τ is theduration of the molding cycle per part, the cooling channels can have asubstantial effect on the temperature of the cavity during a cycle. Fora typical molding cycle of 10 seconds and for a value of α typical ofthat for stainless steel, this distance is approximately 7 mm.

A necessary step in the creation of tooling with conformal coolingchannels is to remove the powder from inside the channels. Powderremoval may be accomplished using vibration of the green mold, which issufficient to flow the unbound powder out of the channels. The channelsbeing open at both ends allows for a variety of removal techniques,including blowing with air or vibration. Another useful technique is toimmerse the printed component in water charged with CO₂ (soda water)within a vessel capable of sustaining a vacuum, as illustrated in FIG.15. The pressure is then rapidly dropped through the use of a vacuumpump and vacuum reservoir, thereby causing the CO₂ to come out ofsolution. The bubbles thus formed eject the loose powder from thechannels.

If infiltration is used to attain full density, inadvertent filling ofthe cooling channels with infiltrant metal must be avoided, as thiswould plug the channels. The cooling channels are in effect very largeinternal pores and can be filled with infiltrant metal by the samemechanism which fills the micro-porosity between the powder particles.One way to avoid channel blockage is to use exactly enough infiltrantmaterial to fill only the micro-porosity between the powder particles.Since infiltrant material will preferentially fill small pores first.,the much larger size of the cooling channels will prevent them frombeing filled.

Referring to FIGS. 10A, 10B, and 10C, the cooling channels 32 can alsoremain free of infiltrant by infiltrating the part 30 placed on stilts34 to raise the part a predetermined distance above the free surface ofa pool of molten infiltrant 36 contained in a crucible 38, which may bean alumina coated graphite. The stilts are preferably fabricated of apowder material similar or identical in composition, size and packingfraction to the material used in the tool itself, such as a stainlesssteel. The infiltrant, such as bronze, wicks up the stilt by capillarityand then into the body of the tool. The infiltration may be performed,for example, in a furnace 40 supplying an argon/hydrogen atmosphere atan elevated temperature, such as 1100° C. By selecting the height of thestilts properly and considering the size of the cooling channel intendedto be kept clear, it is possible to guarantee that no infiltrant entersthe cooling channel. As the infiltrant rises to the level, H, above thefree melt surface of the infiltrant pool, the hydrostatic pressurewithin the infiltrant is lower than in the surrounding ambientatmosphere. This hydrostatic pressure differential is given by:

Pressure Differential=ρgH

where ρ is the density of the molten infiltrant, and g is theacceleration of gravity. The maximum capillary rise in a circularchannel of radius r is given by:$H = \frac{2\quad \gamma \quad \cos \quad \theta}{\rho \quad g\quad r}$

where γ is the surface tension of the liquid (molten infiltrant) and θis the contact angle of the liquid against the walls of the channel.(See, for example, Fluid Flow, Sabersky, Acosta and Hauptmann,Macmillan, 1971, p. 14.) Thus, if the stilt height is designed to ensurethat a cooling channel of radius r is more than the distance H above thefree melt surface, the infiltrant does not fill the cooling channel.

This rule of thumb for determining the needed stilt height is meant asan approximate indication. The exact minimum height needed may depend onthe shape and orientation of the cooling channels. For example, ahorizontal channel may be more resistant to filling than a verticalchannel and the minimum stilt height may thus be less, since the ratioof perimeter to cross section for a horizontal channel is lower than fora vertical channel.

As a typical example, consider a bronze infiltrant undergoingcapillary-driven infiltration of a porous stainless steel skeleton wherethe powder size of the stainless steel is 50 microns in diameter and thepowder is packed to approximately 60% by volume. For a typical bronze,the density of the molten infiltrant is 8800 Kg/m³, the surface tensionis 1 N/m, and the contact angle is close to zero. The capillary risepossible within the body of the perform itself, where the size of thepores between the powder is approximately 10 microns in radius, may becalculated to be 2.3 m. In other words, the capillary rise within aporous body would be sufficient to ensure the full infiltration of asample more than 2 meters tall. In contrast, if the smallest coolingchannel was 1 mm in diameter, the maximum capillary rise would be 22 mm.In this case, a stilt of height greater than 22 mm would be sufficientto ensure that the cooling channel would not fill, while the bulk of thepart would fill.

The stilts may be prepared independently of the part, either by threedimensional printing or by conventional powder metallurgy techniques.Alternatively, the stilts may be created by printing them integral tothe part itself. The stilts provide a further advantage in assuringknowledge of the entry point of the infiltrant into the part to beinfiltrated. It may be desirable to control how the infiltrant frontadvances, and this provides a method to exercise such control.

The stilts also serve an important function in pre-alloying theinfiltrant. Generally, the infiltrant is able to dissolve some of thepowder material, thus causing some erosion of the powder perform. Thiserosion can continue until the concentration of the dissolved powder inthe infiltrant reaches a limit imposed by the particular materials. Ifthe pool of molten infiltrant is placed in intimate contact with thepart to be infiltrated, the part itself will erode as the infiltrationtakes place. However, if the part is placed on stilts, the erosion canbe entirely confined to the stilts themselves. By the time the meltreaches the part, the melt will be saturated and will dissolve no morepowder. While in principal it is possible to pre-alloy the infiltrantwith the powder material so that it is saturated, in practice it isdifficult to get precisely the right concentration, as this pre-alloyingis typically done in a separate furnace operation and the temperaturemay be different than the infiltration temperature. Having thepre-alloying done by the stilts themselves guarantees the rightconcentration in the infiltrant. This is especially the case if thestilts are made of the same powder used to create the part.

The key to maintaining the internal structure clear of infiltrant is toestablish a pressure differential between the ambient atmosphere and theinfiltrating melt front. Creating this pressure differential byhydrostatic head is one option. Another option is to create such apressure differential by the selective application of gas pressure.

This pressure differential can be created by the embodiment illustratedin FIGS. 16A through 16D, which show a perform for a tooling insert 101which is made by three dimensional printing. Such a part might beprinted of tool steel or tungsten carbide powder. The perform has amolding cavity 106 and cooling channels 102. The cooling channels mustbe free of infiltrant in the final product. Toward this end, the partincludes a box 103 with a hole 104 to aid in the infiltration process,as discussed below. The perform may be created with a polymeric binder,debound, and lightly sintered as previously described.

The perform is placed in a crucible 105, for example, an aluminacrucible, in a furnace (not shown). Infiltrant alloy 107 is then placedin the bottom of the crucible. For example, a bronze alloy can be usedin the form of a coarse powder. A tube 109 such as a stainless steeltube is inserted into the hole 104 in the perform 101. The tube, or someextension of it, can extend through the wall of the furnace and besupported by a fitting which seals its outside diameter to the wall ofthe furnace. The furnace is then filled with an inert gas such as argon,and either argon can also be admitted through the tube or a suction canbe applied to the tube so that furnace gases are pulled out through thetube, either method guaranteeing that air is not admitted through thetube. Alternatively, a vacuum can be applied to the furnace if theinternal diameter of the tube is sealed outside the furnace.

The furnace is then heated up and the infiltrant is melted. Theinfiltrant wicks through all of the part 101 and may fill some or all ofthe channels 102, depending on their size and height above the freesurface of the melt. The box 103 only partially fills with theinfiltrant, as it is wide enough to prevent full capillary infiltration.The infiltrant rises to the level 108, slightly higher than the freemelt surface of the infiltrant, the amount above the free melt surfacebeing determined by the width of the box. The infiltrant also wicksthrough the walls of the box 103 and seals the tube 109 to the perform.At this point, if the infiltration was done under vacuum, an inert gasis admitted to the furnace until the pressure is close to atmospheric. Apartial vacuum is now applied to the tube 109 from outside the furnaceusing a small vacuum pump. This vacuum draws the residual molteninfiltrant from outside the part 101 and from within the coolingchannels 102, and the melt level within the box 101 rises until thesetwo sources of molten infiltrant are depleted. The infiltrant does notdrain from the body of the part 101, because the meniscus of the molteninfiltrant attached to the powder can sustain a substantial pressuredifferential across it. The furnace is cooled and the part removed. Thebox 103 and its content of infiltrant alloy can then be removed bymachining.

Another method of preventing infiltrant filling of cooling channelsinvolves the selective printing of an infiltration stop material. Inthis case, two different materials are dispensed through the printheadduring the printing of the part. Referring to FIG. 17, the firstmaterial is a conventional polymeric binder 71 which defines the greenpart geometry and provides the green part with cohesive strength. Thesecond material is an appropriate permanent binder material 72 which isprinted into the regions of the powder bed forming the walls of thecooling channel 73. This binder is intentionally chosen so that itremains in the powder bed during firing and prevents the infiltrantmaterial which will be used during post-processing from filling theseregions by creating a surface on the powder which is not wetted by themolten infiltrant. Such non-wetted agents may be applied to the surfaceof the channel which is to be kept free of infiltrant. Alternatively theanti-wetting agent can be applied throughout the volume of the channel.In both cases, this anti-wetting agent can either remain in the tool orcan be removed from the tool after the infiltration step by etching. Theetching step is suitable to particular formulations of anti-wettingagent material. For example, silica can be etched in hot sodiumhydroxide or potassium hydroxide, which also leaves many metal alloysystems unaffected. The part is removed from the powder bed afterprinting, and loose powder is removed from the cooling channels bygentle vibration. During post-processing, the infiltration stop materialremains resident in the walls of the cooling channels, preventinginfiltrant penetration into the channels.

An example material system for this process is 60 μm stainless steelpowder, a polymeric binder, colloidal silica binder and bronzeinfiltrant. The colloidal silica binder is printed into the channelwalls, locally coating the stainless steel powder and forming aninfiltration stop barrier. The silica also effectively prevents anysintering from occurring in the printed regions during firing. Thesilica binder remains resident during firing and prevents the liquidbronze from wicking into the channel walls. Other ceramic colloids orslurries, for example, colloidal alumina or suspensions of fine silicaor alumina particles, can be used as infiltration stops.

In another variation, the cooling channel area is increased by printingthe conventional binder in the areas which are to become coolingchannels as well. In this embodiment, infiltration stop material is onlyprinted into those regions of the green part which are to act as porouscooling channels after the part has been completely post-processed.After printing is completed, the green part is removed from the powderbed. The cooling channels remain filled with the powdered metal of whichthe rest of the three dimensional printed green part is composed. Duringthe post-processing sequence, the polymeric binder is removed by thermaldecomposition, while the infiltration stop material remains. Afterlightly sintering the green part, the metal skeleton is infiltrated witha lower melting point alloy. The infiltration stop prevents theinfiltrant material from entering the interstices of those areas whichare to become porous cooling channels. In this manner, the coolingchannels are left as porous passages, while the remainder of the toolvolume becomes a fully dense material. The infiltration stop materialcan also be formulated to allow a slight amount of sintering duringfiring, thus allowing the porous channels to be strengthened and resistthe pressure associated with coolant flow through the channel. Anadditional advantage of this method to increase cooling channel surfacearea is that the loosely sintered porous matrix can provide additionalmechanical support and load bearing capacity to resist tool deflectionduring the high pressure cycle of injection molding. Typical coolingchannels, being completely open, can support no mechanical load.

Another aspect of the present invention provides an increase of theeffective heat transfer surface area for a cooling channel of a givennominal diameter by a modification of the wall geometry of the coolingpassage to maximize surface area for a given nominal diameter. FIGS. 6and 7 show cooling channel geometries which have been optimized for highsurface area. Lining the walls of the channel with radial fins 11 orother geometries 12 greatly increases surface area. The application oftextures, such as short stubs 13 or mounds 14, also increases surfacearea. Additionally, irregular textures induce turbulent flow in thecoolant, thereby increasing mixing and overall heat transfer. One mannerof creating these surface geometries is to incorporate them into themold CAD model and then to three dimensionally print the mold. Texturesof varying complexity, such as the inclusions of overhangs andundercuts, are thereby automatically incorporated into the mold coolingpassages.

The ability of the three dimensional printing process to selectivelydeposit different materials throughout the volume of a tool allows forthe creation of areas composed of high thermal conductivity materialembedded in a matrix of high strength and hardness. As shown in FIG. 8,thermal paths 18 are embedded in the tool volume. Such paths may beplaced in regions 18 which are not easily accessed by cooling channels.Since the thermal paths do not extend completely to the inner moldsurface 19, the hardness and abrasion resistance of the mold surface isnot compromised. One method of creating high thermal conductivity pathsis to selectively print a slurry which contains sub-micron sizedparticles of a high thermal conductivity material, such as copper orsilver, into those regions of the tool volume which are to have enhancedthermal conductivity. The copper particles will fill the void spacesbetween the powder particles in the spread layer and become incorporatedinto the finished tool during post-processing. The tool may be sinteredor infiltrated as described above. In this manner, selected regions ofthe tool have increased thermal conductivity. These high thermalconductivity paths can extract heat from near the mold surface anddeliver it to the underlying conformal cooling passages 20. Also,gradual compositional gradients can be incorporated into the tool volumeto minimize local stresses caused by local variations in the coefficientof thermal expansion and to increase the overall toughness of the tool.

Another aspect of the current invention is to create pockets 26 of highthermal conductivity material within the body 27 of the tool, shown inFIG. 13. Such pockets of high thermal conductivity can be created byfirst removing the powder from the region which is to become a pocketand then filling the pocket with infiltrant. For example, if a tool isfabricated by creating a porous skeleton of steel powder and infiltratedwith a copper alloy, the high thermal conductivity of the copper alloywill provide the desired benefit when a pocket is fully filled with thecopper alloy. Such pockets can be buried within the tool so that thesurface of the tool is composed of the harder combination of powder andinfiltrant, while the buried pocket provides a region of high thermalconductivity. As an example, the powder used for the porous skeleton canbe stainless steel or tool steel powder. The infiltrant can be a chromecopper or a beryllium copper alloy, both of which are known to combinereasonable hardness with a high thermal conductivity.

Such buried pockets 26 can be usefully deployed in the region betweenconformal cooling channels 28, shown in FIG. 13, as they will tend toconduct the heat from the regions between cooling channels to thechannels. This creates the challenge of infiltrating a tool so as tofill buried pockets with infiltrant, but guaranteeing that coolingchannels, which might be of similar size or smaller, remain free ofinfiltrant. Referring to FIGS. 14A through 14C, one method to accomplishthis is the following:

1) Print a part 42 by printing a binding agent, but do not print thebinding agent where the cooling channels 44 and buried pockets 46 are tobe located. The cooling channels, by the nature of their function, haveopenings from which powder can later be removed. The buried pockets mustalso be printed with at least one opening to the outside of the part sothat the powder can be removed from inside.

2) Remove the powder from inside the cooling channels and the buriedpockets.

3) Seal the opening of the buried pocket to the outside of the part.This may be accomplished for example by applying a paste of powder andbinder 48, preferably the same powder and binder used to create thepart.

4) Infiltrate the part with molten metal infiltrant 50. In order toguarantee that the buried pockets fill with metal, the part may need tobe immersed in molten infiltrant, shown in FIG. 14B, since the size ofthe buried pocket may prevent it from filling by capillarity, dependingon its height above the free melt surface. This infiltration step isbest performed in a chamber 52 under vacuum conditions at an elevatedtemperature, such as 1100° C., to eliminate the possibility of trappinggas inside the buried pocket, which trapped gas would prevent completeinfiltration of the pocket. During this infiltration step the coolingchannels fill with infiltrant as well.

5) Drain the infiltrant from the cooling channels by placing the part onstilts 54 in a furnace 56, illustrated in FIG. 14C. The cooling channelswhich are open to the outside of the part will admit gas which willallow the channels to drain, illustrated in FIG. 14D; however, theclosed buried pockets will have no such path for the admission of gas,and hence, these channels will not drain. It is possible to combine thisstep with step 4 by performing the submerged infiltration of step 4 withthe part on stilts in a crucible from which the infiltrant can then bedrained, resulting in the draining of the cooling channels.

Other methods are possible for the creation of buried pockets. Forexample, the porous perform can be created as in steps 1 through 3above. A stilt infiltration can then be performed under vacuum whichresults in the infiltration of the body of the part around the buriedpocket, but not the buried pocket itself, as the larger size of thispocket will prevent infiltration. While the infiltrant is still molten,the furnace is raised to a higher pressure, for example 10 atmospheresof argon, which results in filling of the buried pocket by flow of themolten infiltrant into it. The pressure must be sufficient to overcomethe capillary retention of the infiltrant and cause it to flow into thepocket. Another alternative would be to use a high-conductivity materialwith a lower melting point in a second infiltrant.

In some processes, rapid thermal cycling may require a tool to undergo atemperature difference of 100 to 200° C. within 1 to 5 seconds. Oneapproach to implementing fast thermal cycling is to reduce the mold'sthermal mass to provide a region of low thermal inertia and to insulatethe mold from its support structure. As illustrated in FIG. 9, a methodof accomplishing both these tasks is to back the mold surfaces 21 andconformal cooling channels 22 with a cellular structure. Conformalcooling channels can be placed in close proximity to the surface of thetool within a shell 25 of minimum thickness. Typically, the coolingchannels have a diameter of 3 mm and the shell has a thickness of 8 mm.The cellular structure 23 eliminates the majority of the metal mass fromthe mold volume 24 while providing mechanical support to the moldsurfaces. Whereas heat transfer between the mold surfaces and theconformal cooling channels is unaffected by the cellular structure, heattransfer from the cooling channels to the body of the mold is greatlyreduced, because of the decrease in thermal mass and the increase inthermal resistance which the cellular structure brings about. Thecellular structure thermally isolates the mold surfaces and coolingchannels from the body of the mold. The thermal mass which must becycled by the heating/cooling system is now reduced to approximately theshell 25 which defines the mold cavity 21 and encompasses the coolingchannels 22. This substantially reduces the heat transfer requirementsof the heating/cooling system and makes rapid thermal cycling of themold surface possible. The cellular structure 23 may be regular, asshown on the left side of FIG. 9, or may be more irregular to conform tothe configuration of the mold surface and the shell, as shown on theright side of FIG. 9. The cellular structure may have athree-dimensional form, as illustrated in FIG. 20.

Although a cellular structure of this type would be very difficult toincorporate in a standard mold because of the multiplicity of cells andtheir irregular shape, such a structure is readily incorporated into amold made with the three dimensional printing process. Cellular regions,designed to be sufficiently strong to resist pressures of fabricationand operation, are defined in the CAD file and result in unprintedregions in the green part. Unbound powder is removed from the greenpart, resulting in an open cellular structure. A requirement of thestructure, therefore, is that the cells form an open network, so thatthe unbound powder can be removed. The part then may be sintered orinfiltrated to achieve full density. The cellular structure can beconformal in shape to the mold surface and cooling channels to maximizethe reduction in thermal mass and to maximize the structural integrity.The cellular structure can be incorporated into the mold withoutadversely effecting the mold core and cavity geometry or mechanicalproperties.

An added advantage is the ability of such a cellular structure toaccommodate situations in which different members are at differenttemperatures, as happens in normal operation of a rapidly thermallycycled tool. It is known that cellular solids in general exhibit thermalshock resistance which is superior to a solid piece of the bulk materialof which the cellular solid is made. (See, for example, Cellular Solids.Structure and Properties, L. Gibson and M. Ashby, Pergamon Press, p.208.) Fundamentally this improved thermal shock resistance is due to theability of the cellular material to respond by bending. Such increasedresistance to thermal shock may be of particular value in die casting ofmetal parts.

The cellular structure can, for example, be designed as a triangulated,pyramidal truss structure. Such a structure provides a small area forconduction down the length of the truss element and guarantees that thesupport members are, to the maximum extent possible, in a purelycompressive or tensile state of stress, with no bending moments present.Further, the presence of the cross members guarantees that the supportmembers will not collapse by buckling. The minimum cross sections of thetruss elements will be determined by structural requirements in view ofthe loads anticipated during operation of the tool.

Such a structure which combines conformal cooling channels and trussstructures can be created by printing binder where the body of the partand the truss are to be formed and by not printing binder where thecooling channels and region between the truss structure are to beformed. The loose powder is then removed from within the coolingchannels and from around the truss structure. The structure is thensintered or infiltrated by capillarity as described previously. Analternate way to provide a mold with reduced thermal mass is to provideby three dimensional printing a shell 91 with conformal channels 92within it. Rigid and low thermal inertia backing material 93 can then befilled in behind the shell 91 in order to provide the requisite strengthand stiffness, as illustrated in FIG. 18. A suitable filler material isa chemically bonded ceramic or a filled epoxy.

A further aspect of the present invention relates to the control duringuse of the temperature of a tool 60 designed for rapid thermal cycling.In the simplest implementation, shown in FIG. 11, two reservoirs of oilare maintained, one 62 at, for example, 50° C. and the other 64 at, forexample, 200° C. The oils may be continuously circulated by pumps 70 onflow paths 63 and 65 respectively. Prior to injection of the plastic,the hot oil is run via flow path 67 through the channels within themold, the plastic is then injected, and the cold oil is then circulatedvia flow path 69 through the mold's channels to cool the mold. Valving,such as three-way valves 68, may be provided to direct the oil along theappropriate flow paths. The part is ejected and the cycle is repeated. Asuitable high temperature oil for heat transfer applications isMultitherm 503 available from Multitherm Corp. The oils may be movedthrough the mold from the reservoirs using the pumps 70. The oils mayalso be moved by pressurizing pairs of reservoirs 80 a, 80 b, 82 a, 82 bwith gas, as shown in. FIG. 12. In this case, opening the appropriatethree-way valves 84 transfers the oils back and forth between theirrespective reservoirs.

Additionally, it is frequently desirable to exert greater control overthe temperature history of the tool than is possible with just tworeservoirs, one at a high and one at a low temperature. For example,during the cool down, it may desirable to drop the temperature as aprescribed function of time and not all at once in a step change, toobtain the minimum state of residual stress in a part. Also, whenheating the mold, it is not necessary to follow a prescribed timehistory as there is no plastic present in the mold. However, it isdesirable to heat the mold up as fast as possible and to a uniform andwell controlled temperature. If a single reservoir of oil is used, theoil will suffer some reduction in temperature in the plumbing on the wayto the tool. Further, the temperature of the tool can at best beexpected to respond with an exponential rise in temperature whichasymptotically approaches the temperature of the hot oil. Thus, someamount of time will have to pass before the temperature of the tool iseffectively at the temperature of the hot oil. A solution is to exertcontrol over the temperature over time, for example, by providingmultiple reservoirs at multiple temperatures. Referring to FIG. 19, areservoir 121 can be used to provide oil at 210° C. for a short time andthen a switch via appropriate valving 125 can be made to a reservoir 122providing oil at 200° C., the desired temperature of the mold 126 priorto injection. This will provide a more rapid heat up. In cooling down,the flow can be switched among reservoirs 123, 124 each of successivelylower temperature. Any appropriate number of reservoirs can be used.Valves to mix the oils from various reservoirs can also be provided toobtain oil at intermediate temperatures. Another way is to provide foron-demand heating and cooling of the oil as it flows through the system.

Another manner of creating tooling with conformal cooling channels is bythe electroforming process. In this case, a nickel shell can beelectroformed onto a positive form, as is conventional practicedescribed above. The cooling channels can then be formed by depositingonto the surface of the nickel shell a sacrificial material in the formof beads. This material may be a polymeric or wax material, for example,applied by a hot extrusion technique, or it may be a low melting pointmetal, such as a solder. The electroforming process is continued, addingmaterial to the thickness of the shell and over the beads of thesacrificial material. If the sacrificial material is a polymer, it willhave to be rendered conductive through the use of electroless platingtechniques as is well known in the art of plating. This additionalelectroformed material can be copper to provide a higher thermalconductivity. Finally, the sacrificial material is removed, by meltingin the case of the low melting point metal or by melting, dissolving, orburning in the case of the polymeric materials.

The capabilities described above make tooling produced by SFFfabrication techniques such as three dimensional printing superior totooling produced by conventional methods. These capabilities can beutilized in a variety of ways depending on the specific toolingapplication. Also, although the invention is particularly suited forthree dimensional printing techniques and is described in relation tosuch techniques, other methods, such as selective laser sintering, canalso be used to create tooling directly from a computer model. It willbe appreciated that the thermal modifications of the present inventioncan be applied to tooling made directly from a computer model regardlessof the process used. Additionally, the tooling so made can be used for awide range of fabrication processes.

The invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims.

We claim:
 1. A process for fabricating a part made of a moldingmaterial, by means of a molding cycle having a duration of τ per part tobe molded, said process comprising the steps of: a. providing toolinghaving a thermal diffusivity of α, a mold surface and an internal heattransfer channel through said tooling, said channel being spaced fromsaid mold surface a distance less than twice (ατ)^(½); b. providing saidmolding material to said tooling so that it contacts said mold surface;c. providing heat transfer fluid to said heat transfer channel tofacilitate exchange of thermal energy between said tooling and saidfluid during said molding cycle; and d. removing from said tooling at atime interval of τ, a formed part.
 2. The process for fabricating a partof claim 1, said providing tooling step comprising providing toolinghaving a heat transfer channel that is spaced from said mold surface adistance less than (ατ)^(½).